The present invention relates to pressurized water nuclear reactor power plants and more particularly to support arrangements for valves which are provided with the pressurizer to protect the plant against overpressure and thereby provide reliability for safe plant operation.
In a pressurized water nuclear power plant, a pressurizer is employed to provide relief for coolant overpressure. Typically, the pressurizer is a vertical, thick-walled vessel having a hemispherical head and having its bottom end supported by a flanged skirt.
In accordance with applicable code requirements, relief valves must be provided for a first level of overpressure protection and safety valves must be provided for a second level of overpressure protection.
The number and sizes of the protection valves are determined by requirements for the amount and speed of steam dumping. In turn, plant load rejection requirements determine steam dump requirements.
Generally, three safety valves, (typically 6" valves) and three relief valves (typically 3" valves) have been employed to meet steam dump requirements for 100% load rejection in all standard size power plants, i.e. all 2, 3 and 4 loop plants with 12 or 14 foot reactor cores. To provide for 80% load rejection, only two relief valves have normally been provided for the various plant sizes. The usual practice has also been to provide an isolation valve in series with each relief valve to provide for relief valve maintenance.
Normally, a single nozzle is provided in the pressurizer head for common connection to all the isolation and relief valves. However, a separate nozzle is normally provided in the pressurizer head for connection to each safety valve. Thus, plant safety is enhanced by the redundancy inherent in the multiple valve and multiple nozzle system design.
In the prior art, the piping and support arrangement for the relief and safety valves has been plant dependent. Typically, the plant architect/engineer has had the responsibility for designing (1) the piping runs from the nozzles to the valves and from the valves to the common downcomer and (2) the support arrangement for the piping and valve layout.
Architects/engineers have had problems in arranging and supporting the piping and valves to accommodate the large valve discharge forces while complying with allowable pressurizer load limits. In developing plant-by-plant designs for valve support arrangements which do meet overpressure protection performance requirements, a lack of standardization has resulted in a higher cost of plant construction, difficulties in valve maintenance and service, and valve availability and life which are less than may otherwise be possible with a standardized and well-planned valve support arrangement.
Discharge piping layout and design has become a uniquely costly effort especially since it has typically been dependent on the space available after other plant and system design needs have been met. Further, the design of valve support arrangements to meet earthquake protection standards has typically been made more complicated and more costly by structural ties of the support arrangement to the side wall of the containment vessel within which the pressurizer is disposed.
Due to space limitations on the discharge piping, almost every plant has required a different unique layout which cannot be duplicated in other plants. Because of this, excessive amounts of piping has been used to make the structure fit into the containment while meeting performance and structural requirements. Since each valve support arrangement more or less has been a tailor-made structure, costly job-to-job modifications and special analysis requirements have been imposed in the manufacture and installation of pressurizer valve supports. Moreover, additional supports have been needed to satisfy deadweight, hydraulic, thermal and seismic loads and these supports have frequently been tied into the concrete, further complicating the space/interference problem.